In a conventional electronic instrument, the sound waveforms of various instruments are transformed to the numerical waveform data in the pulse code modulation system. The numerical waveform data is stored in a memory and various tones are reproduced based on the numerical waveform data. In the electronic instrument, when almost the actual instrumental sounds or the sound of more various tones are generated, a volume of sound waveforms sampled from actual instruments need to be stored in ROM or other memory in advance. Practically, however, memory cannot be increased limitlessly, and the capacity of memory is restricted in terms of costs.
Various systems other than the pulse code modulation system are known as the system for transforming sound waveforms into numerical data for storage. For example, in the field of information processing, the adaptive differential pulse code modulation system, the vector quantization system or other system is known. In such systems, when sound waveforms are transformed into numerical waveform data, the data is compressed. The volume of data to be stored can be reduced, while the impairment in the quality of the data is minimized.
In the electronic instrument, however, the system for compressing the data for storage is disadvantageous as follows.
The compressed waveform data requires to be processed in use for the reproduction of sounds. The waveform data, compressed and converted according to a specified rule, is computed such that the conversion of the waveform data is inverted, and the inverted waveform data is developed. Thus, the initial waveform data prior to the compression can be reproduced. The development of waveform data takes time. Therefore, even if the development of waveform data is started at the same time when keys are depressed, response time is required between the key depressing and the sounding. Such response time gives the feeling of timing incompatibility to a player. Furthermore, when the performance is played very fast, the sounding fails to follow the key operation.
Consequently, although the vector quantization or other system for compressing data for storage is practical in the fields other than music, such system cannot be applied to the electronic instrument, in which quick response is essential for the data processing.